Many industrial operations are confronted by the build up of static charge on work pieces which then contribute to undesirable particulate contamination, unwanted movement, or other undesirable physical parameters associated with the work pieces. In the preparation of continuous films of shoot plastic materials, extended lengths of non-conductive plastic films pass rapidly over one or more rollers and accumulate substantial electrostatic charge that then attracts surface contaminants, and inhibits tight compaction in take-up rolls, impedes surface coating processes, and otherwise interferes with safe processing of the films, Air ionizers are commonly positioned in close proximity to such moving webs to supply positive and negative ions for substantially neutralizing static charge on the web material. These air ionizers commonly contain pointed ionizing electrodes and operate at voltages of several kilovolts supplied to the ionizer via heavily-insulated cables from remote generators positioned away from the moving web. In large industrial applications, such webs may be several feet wide, operate at high linear speeds, and exhibit wide variations in the amount of static charge requiring neutralization at any given time or location along the moving web.
Typically, ionizing currents of about 0.1 to 10 microamperes per linear inch of the moving web are required for neutralization. The webs may vary in widths from several inches to 20 feet. This requires that the generators which supply such ionizers be capable of sustaining the output current of about 1-5 milliamperes at voltage levels of about 3-15 kilovolts.
Several types of electrical air ionizers are available for controlling static charges on the fast-moving webs. Ionizers that operate at alternating voltages at the power line frequencies of 50-60 Hz are especially capable of efficient neutralization at reasonable cost. Line AC voltage at power-line frequency is applied to a high voltage transformer, the secondary winding of which produces about 4 kV to 10 kV AC voltage at the power-line frequency. This secondary voltage is applied to ionizing electrodes that are commonly positioned within a grounded metal enclosure, with openings through which the electrodes extend. This creates very strong electric field in the vicinity of the electrodes for generating corona discharge. The corona discharge is used to create positive and negative ions in the surrounding air.
These conventional AC air ionizers provide alternating quantities of positive and negative ions around ionizing electrodes closely spaced adjacent to the moving web. Such ions migrate by electrostatic attraction toward the oppositely charged web to neutralize static charge on the web. However, the web will attract the necessary ions of the requisite polarity and the excess ions will return to the electrodes or to the grounded enclosure. In the case of substantially neutral or uncharged webs, ions will not flow to it because of the absence of an electrical field. Operation in this manner provides a condition of self-balancing, and the excess of ions still available after the surface charge is neutralized generally do not cause overcompensation of the original charge on the web. However, in that process there is a considerable loss of the generated ions to ion migration back toward the electrodes when the polarity of the AC voltage reverses. The subsequent ion recombination with the electrodes leaves fewer ions available to neutralize static charge on the moving web and generally reduces the efficiency of such ionizers. Certain known AC air ionizers use two diodes connected to the output of the high voltage transformer to conduct currents of opposite directions and thus serve as half-wave rectifiers for the high voltages supplied through such diodes to ionizing electrodes of opposite polarities. The electrodes are located close to each other to help generate the intense electric field necessary for ionization. This arrangement prevents the electrodes from changing their respective polarities and thereby reduces the loss of ions back to the electrodes that generated the ions. In ionizers of this type, if a web does not carry static charge to attract ions, ions of one polarity generated around an electrode during one half cycle are attracted to and are neutralized at the other electrode of opposite polarity during the subsequent half cycle, thereby providing self-balancing operation. All such conventional ionizers require heavily-insulated cabling between ionizing electrodes and high-voltage transformers mounted remotely from the electrodes because of the large size and heavy weight of such transformers.
Another problem is that such conventional AC ionizers generally are incapable of measuring and monitoring the ionizing currents without employing complex external sensors and circuitry. That difficulty arises from the fact that the alternating potential applied to the electrodes couples capacitively to the electrically grounded components of the ionizer and the generator to produce a significant capacitive current that has a different phase and can substantially exceed the ionizing current. As a consequence, feedback control of AC high voltage ionizers is very difficulty, and the ability to selectively and independently control positive and negative output voltages in AC ionizers can only be achieved using more complex and expensive generator circuitry.
Other known air ionizers of the bipolar pulsed DC type resolve issues of size and weight by using small-size switching generators operating at high frequency. Bipolar pulsed-DC ionizers are capable of detecting the ionization current without employing complex external sensors and accompanying circuitry. For example, the voltage drop across a ground return resistor through which a flow of electrical charges is conducted away from the ionizing electrode can be sensed to provide an indication corresponding to the ionization current. (See, for example, the apparatus described in U.S. Pat. No. 4,809,127). However, this apparatus only monitors its own internal parameters and generally does not respond to charge levels on a moving web or other workpiece. These schemes using pulsed DC voltages of positive and negative polarity supplied to separate ionizing electrodes are also known to avoid the loss of ions back to the electrodes, both by separating the electrodes in space and operating only one electrode at a time, and thus improve ionization efficiency. However, such schemes are limited in pulse repetition frequency due to rise and decay times of the pulses of opposite polarity that tend to overlap at high switching rates. Such ionizers are commonly designed to operate at slow switching rates, typically 5 Hz maximum, to allow the ions to propel away from the ionizer (which is usually installed several feet above the surface to be neutralized) before the electrode of the opposite polarity becomes active and pulls back the ions produced in the previous cycle. Such ionizers generally require relatively large spacings (e.g., 3"-12") between the electrodes of opposite polarities. Circuitry design limitations commonly limit the alternating switching rate of the positive and negative generators to about 5 alternations per second. This low frequency makes pulsed DC technology impractical for neutralization of surface charges on fast-moving webs. Another limitation of these pulsed DC ionizers is the low output power of high voltage generators which are suitable for area ionization purposes, but typically insufficient for neutralization of surface charges on fast-moving webs.
Air ionizers that operate on dual steady-state DC high voltage supplies have found only a limited use for neutralization of surface charge on moving webs. This is due to the difficulty of controlling balanced positive and negative ion generation, and due to the propensity of such ionizers to charge the surface instead of neutralize surface charges. While it is possible to achieve balanced ionization with steady-state dual DC ionizers, considerably higher costs are involved relative to AC ionizers. Devices of the types described above are disclosed in the literature (See, for example, U.S. Pat. No. 5,432,454).